Measuring apparatuses designed for continuously tracking a target point and coordinatively determining the position of said point can generally be combined under the term laser tracker particularly in association with industrial measurement. In this case, a target point can be represented by a retroreflective unit (e.g. cube prism) which is targeted by an optical measurement beam of the measuring apparatus, in particular a laser beam. The laser beam is reflected back to the measuring apparatus in a parallel fashion, the reflected beam being detected by a detection unit of the apparatus. In this case, an emission direction and respectively a reception direction of the beam are ascertained, for example by means of sensors for angle measurement which are assigned to a deflection mirror or a targeting unit of the system. In addition, with the detection of the beam, a distance from the measuring apparatus to the target point is ascertained, e.g. by means of time-of-flight or phase difference measurement or by means of the Fizeau principle.
Laser trackers according to the prior art can additionally be embodied with an optical image detection unit with a two-dimensional, light-sensitive array, e.g. a CCD or CID camera or a camera based on a CMOS array, or with a pixel array sensor and with an image processing unit. In this case, the laser tracker and the camera can be mounted one on top of another in particular in such a way that their positions cannot be altered relative to one another. The camera is arranged, for example, in a manner rotatable together with the laser tracker about the substantially perpendicular axis thereof, but in a manner pivotable up and down independently of the laser tracker and thus, in particular, separately from the optical unit of the laser beam. Furthermore, the camera—e.g. depending on the respective application—can be embodied as pivotable only about one axis. In alternative embodiments, the camera can be installed in an integrated design together with the laser optical unit in a common housing.
With the detection and evaluation of an image—by means of an image detection and image processing unit—of a so-called auxiliary measuring instrument with markings whose relative position with respect to one another is known, it is thus possible to deduce an orientation of an object (e.g. a probe) arranged on the auxiliary measuring instrument in space. Together with the determined spatial position of the target point, it is furthermore possible to precisely determine the position and orientation of the object in space absolutely and/or relative to the laser tracker.
The object whose position and orientation are measured by means of the measuring instrument mentioned therefore need not be a measuring probe itself, for example, but rather can be the measuring aid. The latter, as part of the measuring system, for the measurement, is brought into a position that is mechanically defined relative to the target object or can be determined during the measurement, wherein, by means of the measured position and orientation thereof, it is possible to deduce the position and, if appropriate, the orientation of the measuring probe, for example.
Such auxiliary measuring instruments can be embodied by so-called contact sensing tools that are positioned with their contact point on a point of the target object. The contact sensing tool has markings, e.g. light points, and a reflector, which represents a target point on the contact sensing tool and can be targeted by the laser beam of the tracker, the positions of the markings and of the reflector relative to the contact point of the contact sensing tool being known precisely. The auxiliary measuring instrument can also be, in a manner known to a person skilled in the art, a, for example handheld, scanner equipped for distance measurement for contactless surface measurements, the direction and position of the scanner measurement beam used for the distance measurement relative to the light points and reflectors arranged on the scanner being known precisely. A scanner of this type is described in EP 0 553 266, for example.
For distance measurement, laser trackers from the prior art have at least one distance measuring device, wherein the latter can be embodied e.g. as an interferometer. Since such distance measuring units can measure only relative changes in distance, in addition to interferometers so-called absolute distance measuring devices are installed in present-day laser trackers. The interferometers used for distance measurement in this context use primarily—on account of the long coherence length and the measurement range made possible thereby—HeNe gas lasers as light sources. In this case, the coherence length of the HeNe laser can be a few hundred meters, such that the ranges required in industrial metrology can be obtained with relatively simple interferometer constructions. A combination of an absolute distance measuring device and an interferometer for determining distance with a HeNe laser is known from WO 2007/079600 A1, for example.
In addition, in modern tracker systems—increasingly in a standardized manner—an offset of the received measurement beam from a zero position is ascertained on a fine targeting sensor. By means of this measurable offset, it is possible to determine a difference in position between the center of a retroreflector and the impingement point of the laser beam on the reflector and it is possible to correct or readjust the alignment of the laser beam depending on this deviation in such a way that the offset on the fine targeting sensor is reduced, in particular is “zero”, and the beam is thus aligned in the direction of the reflector center. As a result of the readjustment of the laser beam alignment, continuous target tracking of the target point can be carried out and the distance and position of the target point can be determined continuously relative to the measuring instrument. The readjustment can be realized in this case by means of a change in alignment of the deflection mirror provided for deflecting the laser beam, said deflection mirror being movable in a motorized manner, and/or by pivoting of the targeting unit having the beam-guiding laser optical unit.
For determining the orientation of the measuring aid, a detection direction of the camera is continuously aligned such that an image is detectable in the direction of the tracking beam of the laser tracker. The camera can furthermore have a zoom function, wherein a magnification level can be set depending on the determined distance between laser tracker and target point or measuring aid. With these two adaptation functions (alignment and magnification), the camera can thus continuously detect an image in which the measuring aid and in particular the light points of the measuring aid are imaged. An electronically evaluatable, two-dimensional image of a spatial arrangement of light points arises as a result.
An image processing unit is provided for evaluating the image. This can be used to identify the imaged light points, to determine the centroids of the imaged light points and to determine the image coordinates of said centroids, from which it is possible to calculate for example solid angles between the optical axis of the sensor, in particular the detection direction, and the direction from the sensor to the respective light points.
Such a coordinate measuring machine having a laser tracker and an image detection unit for determining the position and orientation of objects in space on which light points and reflectors are arranged is described in U.S. Pat. No. 5,973,788, for example.
With the use of such coordinate measuring machines, at least three light points that can be registered by the image detection unit and at least one reflector that reflects the measurement beam of the laser tracker are arranged at the object whose position and orientation are to be determined, in positions that are known relative to the object. The light points to be registered by the image detection unit can be active light sources (e.g. light-emitting diodes) or reflectors to be illuminated, wherein the light points are equipped or arranged in such a way that they are unambiguously distinguishable from one another.
WO 2007/079600 A1 discloses a generic laser-based coordinate measuring machine in which a light exit and light receiving optical unit of the distance measuring apparatus, a measuring camera and an overview camera are arranged on a common element, which is rotatable relative to at least two axes, and a laser beam is coupled into the distance measuring apparatus by means of an optical waveguide from a laser module fitted outside the beam directing unit.
By contrast, the measurement of distances without the use of measuring aids having a retroreflector, i.e. measurement directly to a surface of an object to be measured, is not possible with such coordinate measuring machines.
Accordingly, scanning an object surface is not possible either: in order to detect objects or surfaces, use is often made of methods which progressively scan and in the process record the topography of a structure, such as e.g. of a construction site. In this case, such a topography constitutes a continuous sequence of points which describes the surface of the object, or else a corresponding model or a description of the surface. One conventional approach is scanning by means of a laser scanner which in each case detects the spatial position of a surface point by the distance to the targeted surface point being measured by the laser and this measurement being combined with the angle information of the laser emission. From this distance and angle information, the spatial position of the detected point can be determined and the surface can be continuously measured. In many cases, in parallel with this purely geometrical detection of the surface, image recording by means of a camera is also carried out, which, besides the overall visual view, also provides further information, e.g. regarding the surface texture. WO 97/40342 describes a method which records a topography by means of scanner systems installed in a stationary manner. In addition, scanning functions can be integrated into various other instruments as additional functions. WO 2004/036145 discloses, for example, a geodetic measuring instrument which emits a laser beam for distance measurement from its position within the detected range. Such measuring instruments can likewise be modified for detecting surfaces in a scanning fashion, or be operated without modification. One example thereof is motorized theodolites or total stations.
In order to provide such measuring and scanning functionalities that are usable without a retroreflector with generic coordinate measuring machines such as laser trackers, solutions with attachment modules are known from the prior art. By way of example, the document EP 2 620 745 A1 discloses a measuring system consisting of a coordinate measuring machine, e.g. laser tracker, and a scanning module to be fixed thereto.
Measuring distances without the aid of retroreflectors is for example also possible with the distance measuring instruments described in WO 2006/040263 A1 or EP 1 869 397 B1, in which distances are ascertained by means of a frequency-modulated continuous wave radar (FMCW) or a coherent laser radar. However, these solutions lack a target tracking functionality.
However, this multi-component solution is, firstly, complex in terms of production engineering and, secondly, unwieldy and impractical for the user. It would therefore be advantageous to provide a coordinate measuring machine having both a target tracking functionality for a retroreflector and the possibility of ascertaining distances in a manner free of a measuring aid—i.e. in particular without a retroreflector.